Activation of inert or passive metals



United States Patent ACTIVATION OF INERT 0R PASSIVE METALS David Schlain, Hyattsville, and Charles B. Kenahan,

Silver Spring, Md., assignors to the United States of America as represented by the Secretary of the Interior No Drawing. Filed Mar. 15, 1962, Ser. No. 180,052

2 Claims. (Cl. 75-109) (Granted under Title 35, US. Code (1952), sec. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to activation of inert or passive metals for use in displacement of other metals from solution.

A variety of methods have been employed for separating or precipitating a metal from an aqueous solution of a compound of the metal. One such method, known as cementation or displacement, involves treatment of the solution of the desired metal with a precipitating metal which is higher in the electromotive force (e.m.f.) series, resulting in displacement and precipitation of the desired metal. The reaction, in the case of a divalent metal, may be represented by the equation:

where A+ represents ions of the desired metal and B is the precipitating metal.

According to chemical theory, a metal will displace from solutions any metal below it in the e.-m.f. series. However, this reaction is often found to occur at a rate far below normal or sometimes not at all. It is believed that the explanation for this behavior is that the metal surface has become inert or passive to some degree, probably due to the presence of a film of oxide or other inert material on the surface of the metal. An important example of such anomalous behavior is that of nickel prepared from nickel oxide obtained from the Nicaro plant in Oriente Province, Cuba, which, when used as a precipitant for copper from a solution of copper sulfate, shows no reaction (Example 2, Table I). By way of contrast, nickel obtained from the Fisher Scientific Company was quite efiective in precipitating copper under the same conditions (Example 1, Table I). The explanation of this difference in behavior is not known with certainty but appears likely to be due to the presence of impurities in the Nicaro nickel or to some prior treatment such as exposure to elevated temperature, resulting in formation of an oxide coating on the Nicaro nickel.

It is accordingly an object of the present invention to provide a means for activating passive or inert metals for use as a precipitant in a cementation process.

It has now been found that such activation of metals may be accomplished by treatment of the metals with sulfurous acid or sulfuric acid. The precipitating met-a1 may be treated with a sulfurous acid solution prior to addition to the solution of the desired metal, or alternatively, the precipitating metal may be added to the solution of the metal to be precipitated, following which S0 gas is passed through the resulting slurry during part or all of the cementation process. According to a third alternative, concentrated sulfuric acid is used in place of the sulfurous acid in the preliminary treatment of the precipitating metal. It is believed that the probable explanation of the activation is the removal of a film of oxide or other inactive material from the metal surface by reduction in case of the sulfurous acid and by the acid action or possibly S0 formation of the sulfuric acid.

The following examples will serve to more particularly describe the invention.

3,117,000 Patented Jan. 7, 1964 2 EXAMPLES 1-6 In each example, 0.25 gram of mesh nickel powder was used to precipitate copper from 40 ml. of a solution of copper sulfate containing 25 grams CuSO SH O and 1 gram of H SO per liter, the quantities of copper and nickel being stoichiometric. The slight acidity due to the sulfuric acid was found to be desirable but is not essential. Precipitation was effected at a temperature of about 60 C. and the total reaction time was about 10 minutes. Nickel powder purchased from the Fisher Scientiiic Company was employed in Example 1 while the remaining examples used nickel prepared from nickel oxide obtained from the Nioaro plant in Oriente Province, Cuba. The results are shown in Table I.

Table I Nickel Powder Treatment Result Fisher Scientific None 88.5% of Cu precip- Co. itated in 10 minutes.

No reaction during r. 99.3% of Cu precipitated in 10 minutes. 77.6% of Cu precipitated in 10 minutes.

90.4% of Cu precip itated in 10 minutes.

87.1% of Cu precipitated in 10 minutes.

Pretreatment with 5 ($8135 H2SO3(6.4%

Pretreatment with 5 drops concentrated (97%) His 04.

20 m1. HzSOs added to 40 ml. of OuSOt solution.

S02 passed through CusOisolution duriNng reaction with Though the examples employ nickel to precipitate copper, the invention is not limited to these metals but is applicable to any normally active metal which is capable of displacing another metal, lower in the emf. series, from solution. Examples of such active precipitating metals, which may become deactivated or passive, are cobalt, cadmium, tin and iron while the displaced metal may be any metal lower in the e.m.f. series such as silver, gold, platinum, etc. Since the reaction is one of simple ionic displacement, it may be employed with solutions of any soluble salts of the metal to be precipitated such as chloride or nitrate as well as sulfate.

Concentrations and amounts of materials are not critical. The copper sulfate solution may vary from a saturated solution to less than 0.01 gram per liter of copper sulfate while the amount of nickel will generally vary from an approximately stoichiometric amount to any substantially greater amount in the interest of speed and efliciency of reaction. Amounts and concentrations of sulfurous acid and S0 are small, being only enough to react with the surface of the metal, since any greater amounts are of no particular benefit. The length of time of S0 treatment, when employed, is usually only a few minutes. Pressure and temperature are also not critical, room temperature and atmospheric pressure usually being suflicient.

Though the examples employ nickel powder of 100 mesh, the state of subdivision of the precipitating metal is not critical as the reaction will take place with sheet metal or the finest powder. However, the process is usually most rapid when smaller particles of precipitating metal are employed.

It will be appreciated from a study of the foregoing specification that the invention described herein is capable of various modifications and changes without departing from its essential spirit and scope.

What is claimed is:

1. In a cementation process for precipitating copper from an aqueous solution of a copper salt by means of 3 Q nickel, the improvement which comprises activating the References Cited in the file of this patent nickel by treatment with sulfurous acid to remove inacti- V vafing film from the surface of =the nickel. UNITE? STATES PATENTS 2. The process of claim 1 in which the sulfurous acid ,893 Dro llly June 19, 1934 is formed in situ by passing S0 gas through a slurry of 5 2,070,134 Keyes Feb. 9, 1937 the nickel and the solution of the copper salt. 2,189,263 Hampr cht et a1 Feb. 6, 1940 

1. IN A CEMENTATION PROCESS FOR PRECIPITATING COPPER FROM AN AQUEOUS SOLUTION OF A COPPER SALT BY MEANS OF NICKEL, THE IMPROVEMENT WHICH COMPRISES ACTIVATING THE NICKEL BY TREATMENT WITH SULFUROUS ACID TO REMOVE INACTIVATING FILM FROM THE SURFACE OF THE NICKEL. 